Python's slice notation is used to return a list or a portion of a list. The basic syntax is as follows:
start_at is the index of the first item to be returned (included),
stop_before is the index of the element before which to stop (not included) and
step is the stride between any two items.
All three of the arguments are optional, meaning you can omit any of them. For example:
nums = [1, 2, 3, 4, 5] nums[1:4] # [2, 3, 4] (start at 0, stop before 4) nums[2:] # [3, 4, 5] (start at 0, stop at end of list) nums[:3] # [1, 2, 3] (start at 0, stop before 3) nums[1:4:2] # [2, 4] (start at 1, stop before 4, every 2nd element) nums[2::2] # [3, 5] (start at 2, stop at end of list, every 2nd element) nums[:3:2] # [1, 3] (start at 0, stop before 3, every 2nd element) nums[::2] # [1, 3, 5] (start at 0, stop at end of list, every 2nd element) nums[::] # [1, 2, 3, 4, 5] (start at 0, stop at end of list)
As you can probably tell from the examples above, the default values are
start_at = 0,
stop_before = len(nums),
step = 1.
An idiomatic way to shallow clone a list would be using
nums_clone = nums[:]).
All three of the arguments also accept negative values. For
stop_before, a negative value means counting from the end of the list instead of counting from the start. For example
-1 would represent the last element,
-2 the second last element etc. For example:
nums = [1, 2, 3, 4, 5] nums[1:-2] # [2, 3] (start at 1, stop before 2nd to last) nums[-3:-1] # [3, 4] (start at 3rd to last, stop before last)
step means that the list is sliced in reverse (from end to start). This also means that
start_at should be greater than
stop_before and that
stop_before in the context of a reverse stride is more like
stop_after if you are looking at the list non-reversed. For example:
nums = [1, 2, 3, 4, 5] nums[::-1] # [5, 4, 3, 2, 1] (reversed) nums[4:1:-1] # [5, 4, 3] (reversed, start at 4, stop after 1) nums[-1:1:-2] # [5, 3] (reversed, start at last, stop after 1, every 2nd)
Bear in mind that slice notation is very forgiving, so you'll get an empty list if the arguments' values are out of the list's range. For example:
nums = [1, 2, 3, 4, 5] nums[6:8] #  nums[:-10] # 
Learn everything you need to know about Python's slice assignment with this handy guide.
Merges two or more lists into a list of lists, combining elements from each of the input lists based on their positions.
Groups the elements of a list based on the given function and returns the count of elements in each group.